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The Beach Ridges a t Santa, Peru: El Nino,Uplift, and PrehistoryDaniel H. SandweissAnthropolog y Departm ent Cornell University Ithac a N Y 4853

Recent work on the beach r idges a t San ta , Pe ru (9 s l a t i tude) upho lds a n e a r l i e r hypo thes is , bas ed onsedimentary evidence, th at th e r idges were formed by mass ive sediment pulses dur ing rain s associatedwith major incurs ions of the w arm w ate r E l Nino countercurrent . Th e r idges can therefore be used to da temajor El Nino events . Th e al tern ate hypothes is for the Sa nta r idge or ig in ci ted minor seq uent ia l upl i f t asthe causal factor ; th is hypothes is ha s been disproven, though one previous ly unrepor ted upl i f t event atabout 4200 years B.P. ha s been identi f ied a t Sant a . In general , landscape al tera t ion processes such as E lNino f loods and tectonic upl i f t affect hu ma n populat ions , an d accurate chronologies of these eve nts ar enecessary to in terpret th e archaeological record. Geoarchaeological research offers th e key to c ons truct inglandscape al terat ion chronologies, which are a lso of use to geologists for s tudies of ear t hq ua ke predict ion,sedimentation processes, and paleoclimatology.

INTRODUCTIONOn the desert coast of Peru during at least

the last 5000 years, two major, episodic pro-cesses have caused rapid, large magnitude al-terations in the coastal landscape. These pro-cesses, tectonic uplift and massive floodingfrom the torrential rains which often accom-pany the El Nino countercurrent, are drivenby different motors. Both act not only sepa-rately but also synergistically to cause rapidlandscape alterations tha t in tu rn have a tre-mendous impact on human coastal popula-tions. Archaeologists concerned with the his-tory of cultural development on the Peruviancoast must take into account the effects ofuplift and El Nino on the societies they studyand on the archaeological record (Moseley,1983a; Moseley et al., 1981). n order to do so,it is essential to establish the chronology andthe spatial boundaries of the major events.

Recent work on the beach ridges to thenorth of the Santa River a t about 9 southlatitude (Figure 1) upport an earlier hypoth-esis that these ridges were formed by majorEl Nino events and can therefore be used todate such events (Sandweiss et al., 1983).Evi-dence is also found for a previously unreport-ed episode of tectonic uplift.

Uplift, El Nino, and ArchaeologySubduction of the Nazca oceanic plate be-

neath the South American continental platemakes the western margin of South Americaone of the most tectonically active areas of theworld. From about 2 to 15 south latitude(approximately the a rea affected by strong ElNino events), the Nazca plate is subductingeastward under South America at an angle ofabout 10 (Barazangi and Isacks, 1976,1979;Jordan et al., 1983).One effect of this shallowangle of subduction is a particularly highlevel of seismic activity. This activity hasfound expression during historic and prehis-toric time in the form of both gradual andcataclysmic coastal uplift (Moseley, 1983a,b;Moseley et al., 1981).A number of mid t o late Holocene upliftevents have now been reported for variousparts of the Peruvian coast (e.g., Craig andPsuty, 1971; Moseley, 1975; Moseley et al.,1981; Pozorski and Pozorski, 1979; Sandweisset al., 1981; Sandweiss et al., 1983).It appearsthat uplift events on the Andean coast aregenerally limited t o local tectonic blocks,probably bounded by major faults runningperpendicular t o the shoreline and controllingriver valley location (Sandweiss et al., 1983:

Geoarchaeology: An In tern at io nal Jou rnal , Vol. 1 , No. 1 , 17-28 (1986)019 86 by John Wiley Sons, Inc. CCC 0883-6353/86/010017-12 04.00


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THE BEACH RIDGES AT SANTA05 80 75O

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Figure 1. Location of study area.279-280). In order to assess the effect of up-lift on cultural development along the Peru-vian coast, it is necessary to elaborateHolocene uplift chronologies for each segmentof the coast: a generalized uplift chronologyhas no validity.El Nino events occur on the order of aboutonce every seven years, though no rigid peri-odicity has been observed. Events have beenclassified according to intensity as normal,abnormal, and very abnormal (Wilson, 1981).A fourth category of extremely abnormalevents would include those occurrences whichcause significant landscape alterations, suchas the recent 1982-1983 event, the 1925event, and the prehistoric mega-Nino ofabout 100 A.D. (Moseley, 1983b; Moseley etal., 1981;Nials et al., 1979).Extremely abnor-mal El Niiios are accompanied by torrentialrains, which cause massive flooding of theotherwise rainless Peruvian coastal desert; itis the flooding, in turn, which is the effectiveagent of geomorphic change. The drainage

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system of the unvegetated Peruvian coast isonly activated during these flooding events.Destabilization of the drainage system by tec-tonic uplift prior to a major El Nino eventtherefore increases the potential of the eventto produce radical landscape alterations(Moseley, 1983a; Moseley et al., 1981).

Strong El Nino events affect the entirecoast down to the Paracas Peninsula at 14south latitude (Cromie, 1980). El Nino cur-rents start in the north and move southward.The effects of any particular event are mostsevere in the north; duration and intensitydecrease to the south (Wilson, 1981; Zuta etal., 1976). Duration may exceed one year, butat the level of the best possible geomorphic orarchaeological temporal resolution, the ef-fects of El Nino a re synchronous over the en-tire affected area.Documentary evidence for historic El Ninoevents covers only the 450 years since theconquest of the Andean area in 1532 (Quinn etal., 1978). Little information is available on

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THE BEACH RIDGES AT SANTAth e prehistory of El Nino. Rollins e t al. (1986)pos tu la te th a t E l Nino may not have s ta r tedunti l about 5000 years ago, approximately atthe time that sea level stabilized followingpost-glacia l eusta t ic r ise . C raig and Shim ada(1986) report on n orth coast flood depositswhich record some El Nino events, whileNials e t a l . (1979) and Moseley e t a l . (1981)found evidence for a ser ies of El N ino floodsculmina t ing in a massive erosion event in t heMoche valley at 8 south la t i tude at about1100 A.D. In the Ch incha valley (abou t 13 25south latitude), I found channel fil l depositswhich incorporate Late Intermediate /LateHorizon potshe rds (ca. 1500 A.D.); th e flood-ing episode which produced those depositsprobably took place shortly after the Con-quest, around 1500 A.D. Richardson (1983:275) has noted t ha t th e Colan beach ridges atabou t 5 sou th latitude were formed from cob-bles tha t could only have entered th e l i t toralsystem during El Nino related floodingevents. Finally, as mentioned previously,Sandweiss et al. (1983)have suggested thatthe Santa beach ridges were deposited as aresu lt of major El Nino eve nts.THE SANTA BEACH RIDGES

The S an ta beach ridges extend for 20 km tothe n orth of the Sa nta River, forming for themost pa r t a windswept, bar ren pla in (Figures2 and 4). ight m ajor ridges can be seen onthe ground (Figures 2 and 31, though they a reharder to discern on the aeria l photographs(Figure 4).Some of the ridges are probablycompound struc tures . The ridges form a t ime-transgressive sequence, with the earl iestridge to th e ea st (in land ), and progressivelylate r ridges to the west. Shell mate rial incor-porated du ring th e formation of the e arl iest ,easternmost ridge (ridge 8 has been C-14dated at 4235 2 115yea rs B.P. (uncorrected)(Sandweiss et al. , 1983:286). This date wasru n on a n agg regate sample of small gastro-pods found in association with opercula in a nunreworked deposit (in th e layer of roundedcobbles visible in Figu res 9 an d 10 of Rollinset al. , 1986, th is issue).The south ern term inus of the m ajor ridge

system is about three k m to the no r th of themodern river mouth. To the east of the ridgesystem is the upl if ted Ostra bayfloor andbeach, with a n associated w arm-w ater mol-luscan faun a tha t ha s major implicat ions forthe organization of the East Pacific Oceancurrent regime pr ior to about 5000 yea rs (Rol-l ins e t a l . , 1981; Roll ins e t a l ., 1986; Sand-weiss e t a l . , 1 983) . Carbon-14 ana ly s i s ofshe l l s f rom a midden assoc ia ted wi th theOst ra beach h as da ted th e s t randing of th i sfossil bay at about 5000years B.P. (Sandweisset a l . , 1983:280), about 800 years before theinitiation of beach ridge formation.Aeria l photographs (Figure4 learly showthe r idges s ta r t ing at t he i r sou the rn t e rminusfrom two foci-an ea ste rn , inl an d, older focusassociated with the presumed earl iest SantaRive r mouth (mouth l ) , a nd a western,seaward, more recent focus associated withthe presumed second river mouth (mouth 2,see Figures 2 and 4) (Sandweiss et al., 1983:285). Th e modern Santa River mouth is lo-ca ted three km to the south of mouth 2 and ha sonly a few, incipient ridges associated w ith it .These incipient s t ructu res ar e not pa rt of theeight r idge system discussed in this paper,nor were referents avai lable to determinewhe ther or not the m ost recent of the m odernridges was formed by th e 1982- 1983E l Ninoevent . River mouth migrat ion was probablycaused by til t ing of the local tectonic blockassociated with Holocene uplif t in th e S an taarea (Sandweiss e t a l ., 1983).

The Beach Ridge-El Nino HypothesisA borrow pi t i n r idge 8 revealed a diagnostic

sedim entary sequence in which a thick layer[ca. one m eter] of poorly sorted, relatively finegrained [sand-sized] materia l with a fewpoorly rounded la rge cla sts was overlain by awell sorted layer of well rounded cobblesabout 80 cm thick (Sandweiss e t a l ., 1983).Figure 9 in Roll ins et al. (1986, this issue)shows th e interface between these two layer sin th e borrow pi t . No bedding is visible.First presented by Sandweiss et al. (1983:286 87), th e following scenario explains the

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THE BEACH RIDGES AT SANTA

8 50

8 55

2

78 40Figure2. Map of study area, showing the Santa beach ridge transect, the Ostra site, collectingstation, and fossil beach, the borrow pit in ridge 8 from which the dated shell sample wasremoved, and the locations of former river mouths 1 and 2.

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THE BEACH RIDGES AT SANTA2Inw e

O YO Fx uL

4R I D G E

Y I GE R I D G E1

R I D G EI D G E

R I D G E RIDGEm u 4 -

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w m 1 1 1 1 I I I I I 1 1 1 I 1 l 1 1 1 1 1 1 1 I A

Figure 3. Diagrammatic illustration of San ta beach ridge profile; ridge crests a re indicated by arrows.

Figure 4. Aerial photograph of the Santa beach ridge plain. The two former river mouths/ ridge foci aremarked.

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THE BEACH RIDGES AT SANTAEl Nino origin hypothesis for the Santa beachridges:

1. A strong El Nino event accompanied by abnormalprecipitation washed large quantities of loose mate-rial into the river from the normally inactive,unvegetated, arid region of the lower Santa drainagebasin, while simultaneously increasing the compe-tence of the river. Imbalances in the drainage systemcaused by tectonic episodes . . . would have increasedstill further the amount or material available fortransport. The normal lack of rains means that thedrainage system can only adjust itself. . . o landscapealterations of tectonic origin during the rare El Ninoevents accompanied by heavy rains. . .2. The rapid efflux of material out of the river mouthcaused rapid progradation of the coast t o the north ofthe river as the material was transported and depos-ited by the longshore current. This material was es-sentially unmodified and poorly sorted, similar to thelower layer found in the beach ridge.3. After the El Nifio rains ceased and the coastalsystem was no longer swamped with sediment, waveaction again dominated coastal processes. The resultwas that fine sediment was removed and larger clastswere rounded and thrown up on the shore as a lag-typedeposit. After each ridge reached a threshold height,material no longer reached the top and a new ridgeformed on the seaward side with the next El Nino.It should be stressed that in this model, a

two-step process is involved in forming theridges. First, El Nino flooding supplies abun-dant sediment t o the littoral regime. Second,normal long-shore processes, perhaps intensi-fied as a result of oceanographic processes re-lated to El Nino, distribute the sedimentalong the shore and form the ridge. This sec-ond step probably occurs very rapidly;Moseley (personal communication) observedthe formation and subsequent erosion of alarge delta at the mouth of the Jequetepequeriver on the Peruvian north coast during theshort span of the 1982-1983 El Nino. Thecoastal winds that drive the longshore currentto the north remain unchanged during ElNino (Cane, 1983:1192),despite the fact thatthe coastal current offshore shifts directionand flows southward. However, El Nino isaccompanied by a rise in sea level along thewest coast of South America (Cane, 1983:1190 191) that may intensify the ridge for-mation process.

An alternate Santa beach ridge formationhypothesis cites sequential minor uplift as thecause of ridge construction (Grolier et al.,

1974;Sandweiss et al., 1983:285-287). Underthis hypothesis, each ridge would represent aformer beach lifted out of contact with thelittoral environment by a vertical tectonicmovement, leading to the formation of a newbeach at the seaward base of the previousfeature. One clear-cut test of the uplift hy-pothesis is that , if correct, the ridges must riseincrementally and sequentially inland, like astaircase. In other words, the ridge which isoldest and furthest inland will have experi-enced the uplift that created it and the cumu-lative effect of the succeeding uplifts thatformed the other ridges; therefore, the oldestridge will be highest. Should this situationnot obtain, then sequential uplift can be elim-inated as the origin of the ridges, though up-lift may still be involved in determining theamount of sediment available for ridge forma-tion by creating imbalances in the drainagesystem.

The only other large magnitude, episodicprocess operating along the Peruvian coastduring the timespan for the Santa beach ridgeformation 4200 B.P. t o present) is El Ninoand i ts associated phenomena. Elimination ofthe uplift formation hypothesis would providestrong support for the El Nino hypothesisoriginally derived from the sedimentologicalevidence.

The Santa Beach Ridge ProfileDuring January of 1984,I leveled a transect

across the Santa beach ridge plain from thehigh tide line a t the modern shore to the Ostracollecting station. This transect is perpendic-ular to the long axes of the ridges. Eight majorridges were defined; Table 1 lists the distancefrom the shore and the height of each ridgecrest relative to the modern high tide line.

Several conclusions are immediately appar-ent from the table and from Figure 3. First,the ridges fall into two groups by height andintervening distance. Ridges 1 6 range from2.27 m to 2.95 m above modern high tide,with a mean height of 2.57 m and a standarddeviation of 0.24 m. Ridges 7 and 8 average3.93 m above high tide, more than five stan-dard deviations away from the ridge 1 av-

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THE BEACH RIDGES AT SANTATable I. Dis tance f rom m odern shorel ineand he ight above modern sea level of t h eSa nta beach r idge cres ts .

Ridge no. D i st an c e ( m ) H e i g h t ( m )1 143 2.272 308 2.373 515 2.544 875 2.955 1148 2.696 1365 2.627 1631 4.088 2504 3.78

Table 11. Dis tance betweenridge crests .Cre s t -Cres t Dis tance (m)

1-0 (shorel ine)2- 13-24-35-46-57-68-7

143165207360273217266873

erage. This difference is best explained by anuplift event of between 1and 1.5 m of verticalmovement following the formation of ridge 7and prior to the formation of ridge 6. Suchrapid vertical movements are well known inthe Holocene record of coastal Peru (e.g.,Craig and Psuty, 1971; Pozorski and Pozorski,1979; Sandweiss et al., 1983). Nevertheless,sequential uplift cannot account for the for-mation of the ridges; neither ridge group (1 6and 7-8) shows the necessary trend of inlandrise.I noted above that the San ta beach ridgesemanate from two foci at their southern ter-minus (Figures2 and 4 , interpreted as formerriver mouth locations. The older, northern fo-cus apparently correlates with the older groupof ridges (7-81, while the more recent focus iscorrelated with the younger ridge group(1 6). The uplift event tha t occurred betweenridges 6 and 7 therefore probably also causedthe shift in river mouth location. Ridge8 hasbeen C-14 dated to about 4200 years B.P. (seeabove); therefore, the uplift event followingthe formation of ridge 7 occurred sometimeafter tha t date. This uplift may be the sameevent that stranded the fossil bay20 km northat the Salinas de Chao shortly after about4000 B.P. (Sandweiss et al., 1983:290).A final point about the beach ridge profiledata concerns the distance between ridges.Table 2 and Figure 3 show that the averagedistance from ridge crest to succeeding (west-ward) ridge crest in group 1 1 6, pairing theshoreline with the ridge 1 crest) is signifi-cantly less than the distance between ridges 7and 8. The following factors might havecaused the variation in inter-ridge distance

between the two ridge groups: 1 ) The amountof time between formation events. 2 The in-tensity of formation events. 3 ) The amount ofsediment available for transport in the Santadrainage basin.

If part of the inter-ridge distance wasformed during the intervals between El Nino-associated ridge formation events, then thisdistance may be proportional to the time be-tween ridge formation events (factor l ) ,as-suming that deposition between events is con-stant . If future field work shows th at time andinter-ridge distance are not correlated, thenfactors 2 and 3 (above) must be considered.The correlation between ridge height abovesea level and the distance from ridge crest tothe next youngest (westward) crest providessome support for the idea tha t inter-ridge dis-tance is directly controlled by the ridge forma-tion process and is not a result of inter-ridgedeposition by normal littoral processes oper-ating between ridge-forming sediment pulses.Using the modern shoreline t o pair with ridge1crest height and ignoring the ridge 7 t o ridge6 height/distance pair because the uplift afterridge 7 formation would have altered the cor-relation, the ridge heighthnter-ridge distancecorrelation coefficient is r2 96.3% (adjustedfor degrees of freedom). The correlation iseven better when the natural logs of crestheight and distance from crest t o succeedingcrest are used: r2 99.5% adjusted for de-grees of freedom (Figure 5). Although thesample is extremely small (seven data pairs),the correlation is so strong that it is hard toignore. Furthermore, when crest height ispaired with distance to the preceding (east-ward) crest, the correlation is significantly

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THE BEACH RIDGES AT SANTA1.36

1.22

I 08

0 .94

0.804.50 5.00 5.50 6.00 6.50 7.00

Figure 5. Regression of ridge crest height on inter-ridge crest to suc-ceeding crest) distance. Data from Table 1 and 2. Y natural log ofcrest height; X natural log of inter-ridge distance.

N A T U R A L L O G OF I N T E R - R I D G E D I ST A NC E S X )

less (r2 78.7% adjusted for degrees of free-dom, data converted to natural logs).When the ridge 7 crest height is paired withthe distance between the crests of ridges 7 and6, converted t o natural logs, and added t o theother data pairs from the crest heightkrest tosucceeding crest distance set, the correlationis lost r2 46.6%, adjusted for degrees offreedom). This fact agrees with the evidencecited above for an uplift following the forma-tion of ridge 7. The fact that the other ridgecrest heights correlate so well with the crest tocrest distances therefore supports the conclu-sion that the ridges were not formed by se-quential uplift.If the intensity of the formation process(factor 2) is involved in the variation in inter-ridge distance, then the earliest El Ninoevents recorded by the Santa beach ridgeswere also the strongest. This conjecture, inturn, would imply that the maximum inten-sity of the El Nifio phenomenon has decreasedthrough time.

If the amount of sediment available fortransport and incorporation in the ridges (fac-tor 3) is a major factor in determining thedistance between ridges, then the data implya decrease in available sediment throughtime. This assumption is supported by the cur-rent lack of significant ridge formation. It ispossible, however, that the relative inactivityof the Santa beach ridge plain in the recentpast is due to some global process not yet un-derstood. Tanner and Stapor (1971:232)stud-

ied beach ridges throughout North and SouthAmerica and found that in almost every case,the beach ridge plains are either being erodedor are stable, and that very few beach-ridgeplains are now aggrading.

The greater apparent availablity of sedi-ment at the time when the Santa beach ridgeplain began to form suggests that the sourcearea underwent a greater amount of tectonicdestabilization between rainfall events thanat any time following the formation of ridge 7 .This early destabilization may have been dueto the uplift event that stranded the Ostrabeach. The distance between ridges 6 and 7,however, is only 266 m, close to the mean ofthe later ridge group and only about a third ofthe distance between ridges 7 and 8. Clearly,destabilization due to an uplift such as th atwhich occurred after the formation of ridge 7is insufficient by itself to account for the dis-tance between the earliest ridges: a greatertime factor is implied. If the El Nino phenome-non began after about 5000 B.P. (Rollins etal., 19861, ridges 7 and 8 record the first majorEl Nino events and the distance betweenthem may reflect the amount of sediment ac-cumulated in the period prior to the initiationof the El Nino phenomenon.Comparison with Other Areas

The Santa beach ridges are different in sev-eral w a y s from ridges reported elsewhere.First, published reports do not mention a sedi-

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THE BEACH RIDGES AT SANTAmentary sequence similar t o that of the Santaridges. Most beach ridge sets are made of well-sorted sand (e.g., Parana, Brazil: Bigarella,1965; Nayarit, Mexico: Curray and Moore,1964; Golfo de Venezuela: Tanner, 1971;Kiawah Island, South Carolina: Barwis,1978; New South Wales, Australia: Thom etal., 1981), although some are composed oflarger clasts. The Chesil beach in England ismade of pebbles, again showing regular sort-ing throughout, and only one ridge is presentinstead of a series (Carr, 1971, 1974; Carrand Blackley, 1974).

Second, the beach ridges reported in theliterature occur in areas of more constant sed-iment supply than the coast of Peru, and theyusually occur in sets. The total number of midto late Holocene ridges in each area is usuallyin the hundreds. Tanner (1971:215) reports50- 100 ridges for the Golfo de Venezuela;Missimer (1973: 387) reports 240-300 ridgesdeposited during the last 4300 years on Sani-be1 Island, Florida; Curray and Moore (1964:81) ound about 250 sand ridges on the coast ofNayarit, Mexico. There is a clear discrepancyin number of ridges between the Santa beachridge plain and other ridge sets. This discrep-ancy supports the hypothesis of Santa ridgeformation as the result of low frequency (epi-sodic), large magnitude events, rather thanby continuously operating processes such asCurray and Moore (1964) suggest for Nayarit.Assuming that the El Nino ridge origin hy-pothesis is correct, the Santa ridges recordonly the strongest El Nino events-about twoevery 100 years. Some of the Santa ridgesmay be compound structures built by severalsuccessive events; however, the two per mil-lenium periodicity is acceptable for mega-Ninos. Moseley et al . (1981) found evi-dence for such large magnitude events in theMoche valley at about 500 B.C., 500 A.D.,and 100 A.D. Judging by the destruction theywrought, these events seem t o be significantlylarger even than the major modern eventssuch as 1925 and 1982-1983.

One beach ridge set does correspond to theSanta set in number of ridges. Richardson(1983) eports nine ridges a t Chira, on the farnorth coast of Peru at about 5south latitude.

These ridges began forming about 5000 yearsago and are composed of sediment grains ofuniform size that have the same constituentsas the Chira river alluvial deposits (Richard-son, 1983:269). Chira ridge formation appar-ently terminated with a tectonic uplift. Justsouth of the Chira ridges, Richardson men-tions another set of ridges (number and widthunspecified) at Colan composed of cobbleseroded from the Talara Tablazo. According t oRichardson, the only mechanism of massiveerosion in this desert region is the El Ninosand there is a distinct possibility that theColan ridges were formed by . . .catastrophicEl Ninos (Richardson, 19839751, from sedi-ment dumped into the coastal system by ElNino floods and worked into ridges throughlittoral processes.The progradation of existing beach ridgeplains apparently began after sea level stabi-lized in the Middle Holocene, about 5000years ago. Progradation rates, where theyhave been calculated, ar e all roughly equiva-lent for these modern ridge plains. The Santabeach ridge plain is about one km across at itssouthern terminus, but it fans out to about 2.5km opposite the Ostra site (Figure 2). Theentire ridge plain formed in the last 4200years, giving a minimum progradation rate ofabout 25 m per century at the southern termi-nus and maximum rate of approximately 60mper century at the widest point. The Chiraridges (Richardson, 1983)prograded at a verysimilar average rate of about 55 m per cen-tury. Missimer (1973:388) derived an averageprogradation rate of 2 .5 t o 5.0 feet per year forridge formation on Sanibel Island, equiva-lent to 75- 150 m per century. Extrapolationfrom the data presented by Curray and Moore(1964) for the Nayarit coast yields rates be-tween 100 and 340 m per 100 years, assuming5000 years of beach ridge development. Thecombination of a tectonically unstable coast-line and a lack of radiometrically datable ma-terial made i t impossible for Tanner (1971) tocalculate progradation ra tes for the Golfo deVenezuela. Other authors do not address theproblem of progradation rates, nor do theyprovide the information necessary to extrapo-late such rates. From the small available

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THE BEACH RIDGES AT SANTAsample, it appears tha t the Peruvian progra-dation rates are on the same order of magni-tude but about one half as large as in otherareas.LANDSCAPE ALTERATION, EL NINO,AND PEOPLE

Landscape alteration and El Nifio areamong the natural processes that have had asignificant impact on human life on thePeruvian coast. Tectonic uplift has altered re-source locations and forced shifts in settle-ment pattern (Sandweiss et al., 1983) and hascaused large-scale prehistoric irrigation sys-tems to fail (Moseley et al., 1981). Coastalprogradation by beach ridge formation in theSanta valley apparently caused abandonmentof a long stretch of formerly occupied shore-line (Sandweiss et al., 19831, though in theChira region further north, the sequentiallyformed ridges continued to be utilized byshellfish collectors (Richardson, 1983). Thisdifference may result from the fact that in theSanta valley, ridge formation follows a signif-icant shift in available molluscan (and proba-bly other marine) resources due to climatechange. In the Chira area, the same molluscscontinue to be available for exploitationthroughout the sequence of ridge formation.Flooding associated with major El Nifioevents caused severe destruction at importantsites such as Galindo in the Moche valley(Moseley, 1983b), and presumably washedaway many irrigated agricultural fields. Thewarm water El Niiio countercurrent has dele-terious effects on the cool water marine faunaof the Peruvian coast, and major events cansignificantly reduce available resources suchas shellfish (Rollins et al., in press) and fish(Barber and Chavez, 1983). Both uplift (Feld-man, 1980) and El Nino (e.g., Osborne, 1977;Sandweiss, 1982; Wilson, 1981; Yesner, 1980)have been invoked as motors n the develop-ment of early complex society on the Peruviancoast.

My own research in northwestern Hondu-ras in 1981 has shown that rapid landscapealteration can have a significant effect onthe composition of small scale societies

(Sandweiss, unpublished manuscript). In anarea where traditional land tenure is basedoncontrol over the crops o r foodbearing trees a nindividual or family has planted, the erosionand reformation of coastal landforms restruc-tured the pattern of landholding and allowedoutsiders t o move into a formerly closed com-munity. A later return to an erosional regimeis returning the community to its formerstructure, but with some of the outsiders nowincorporated into the community. Though thesocial effects of rapid landscape alterationwill be difficult to identify archaeologically,they must still be considered along with theeconomic consequences when such alterationsare encountered in association with the ar-chaeological record.CONCLUSIONS

Sedimentological and geomorphic datastrongly support the hypothesis that floodingassociated with major El Nino events suppliedmassive, episodic pulses of sediment that intu rn caused the formation of the Santa beachridges. The alternative hypothesis of sequen-tial minor uplift as the ridge-building motorhas been disproven. Radiocarbon dating ofmarine molluscs incorporated in the sedimen-tary deposits of the ridges during the forma-tion process can therefore yield a chronologyof major El Nino events on the Peruvian northcoast, while growth increment analysis of thesame molluscs (Rollins et al., in press) couldconfirm the presence of El Niiio events. Theresults from such a study could be checkedagainst other potential means of dating pre-historic El Ninos. Handler (1984)has shown apossible correlation between large magni-tude, low latitude volcanic eruptions and theonset of El Niiio events (he also found anegative correlation of El Niiio with highlatitude eruptions). A recent study of acidityprofiles in Greenland ice cores (Hammer etal., 1980) has demonstrated tha t large North-ern Hemisphere eruptions can be reliablyidentified and accurately dated during theHolocene Epoch. A combination of ice coredata with Handlers findings might be used toretrodict prehistoric El Niiio events (Steve

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THE BEACH RIDGES AT SANTASoter, personal communication). The Thomp-son et al. (1984)study of the Quecaya ice capin southern Peru has clearly identified the1982-1983 and 1976 El Nino events in avarved ice sequence; they sta te that the poten-tial for reliable dates on El Nino events ex-tends back 1500 years, which would overlapwith the upper end of the Santa beach ridgesequence. Geomorphic evidence of floodingevents in combination with archaeologicaldata has already proven successful in identi-fying some past El Nino events see above).In summary, a variety of approaches in-cluding further study of the Santa beachridges can help establish a chronology of ma-jor El Nino events. Such a chronology is of useto climatologists trying to understand the his-tory and functioning of the global climate sys-tem of which El Nino is a part. Knowledge ofthe prehistory of El Nino and of other rapidenvironmental alterations such as coastal up-lift is also essential for archaeologists trying tounderstand the prehistory of the Peruviancoast, or of any area in which such processesare active.

Many colleagues and insti tutions aided me in thefieldwork and in th e thinking rep resented by this paper.Michael E. Moseley first sent me to the Sa nta beachridge plain and provided both logistical support andadvice. The original study w as carried out w ith HaroldB. Rollins, Ja m es B. Richardson 111,and Jack Donahue ,all of whom had significant input in this w ork. Alfred0Narvae z accompanied me on all of the expeditions, and Icould not hav e done th e beach ridge profile witho ut hishelp. Ar thu r L. Bloom ha s provided invalu able guidancein coastal geomorphology, wh ile discussions w ith AlanK. Craig have been inst ru men tal in c lar i fying my think-ing on man y points. The review ers of this paper m adema ny useful sugge stions, qui te a few of which I hav eincorporated in th e f inal version. My financial supportfor the original fieldwork came from National ScienceFound ation and Fulb right fellowships; surve ying of thebeach ridge profile was sup porte d by a Grant-in -Aidfrom the Cornell Chapter of Sigma Xi. I would like tothan k al l of these individuals and inst i tu t ions, thoughI claim all errors a nd omissions as my own.

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